Field of the Invention
The present invention relates to a new multifunctional initiator, soluble in apolar medium and resulting in the synthesis, by anionic route, of well defined star-shaped polymers, especially of star-shaped polymers comprising first-generation arms made of vinylaromatic or diene polymer blocks, and second-generation arms made up of vinylaromatic or diene polymer blocks or else of two homopolymer blocks, one, joined to the core, being of diene or vinylaromatic type and the other, Joined to the preceding one and different from it, being of vinylaromatic or (meth)acrylic or vinylpyridine type, it being possible for the end groups or the end blocks to be also functionalizing groups.
Star-shaped copolymers with elastomer and thermoplastic branches are thus obtained, it being possible furthermore for the elastomer and thermoplastic blocks to exist in the same arm. Such copolymers have a good heat resistance and are useful as pressure-sensitive adhesives and as agents improving the impact strength and the heat resistance of polymers. As examples of these copolymers it is possible to mention those with cis-poly-1,4-butadiene or polyisoprene blocks, which are good elastomers, and with poly(meth)acrylic end blocks. Such copolymers generally cannot be obtained by a sequential route; in fact, a living poly(methyl methacrylate) block cannot initiate a polybutadiene block.
The invention also relates to the process for the manufacture of the abovementioned multifunctional initiators and to the star-shaped polymers which they make it possible to obtain and to the manufacture of the latter and their applications.
With regard to the anionic synthesis of star-shaped macromolecules, three methods have been developed so far, each of which has specific advantages and disadvantages:
(A) In the "arm-first" method a living precursor polymer (polystyrene, polydiene) is prepared and is used to initiate the polymerization of a small quantity of a diunsaturated monomer, especially divinylbenzene. Small cores of polydivinylbenzene are then formed, surrounded and protected by the precursor chains which have contributed to their initiation. This method produces well defined star-shaped macromolecules, of moderate polydispersity, but it prohibits the functionalization of the arms at their outer end. It can be carried out in polar (tetrahydrofuran) or apolar (benzene, cyclohexane) medium and case therefore be applied to the polydienes, in the case of which the control of the cis/trans 1,4 microstructure involves the use of an apolar medium.
Although this method has been known and applied for a long time, the kinetics of formation of the stars and the proportion of residual double bonds (not affected by the polymerization) in the polydivinylhenzene cores still remained to be clarified, among other things. The present inventors have conducted experiments which have made it possible to reveal the gradual and not immediate formation of the cores and to follow the development of their functionality and the presence of residual unsaturations in these cores, especially in the case of the styrene-divinylbenzene system in a benzene or cyclohexane medium.
Another series of experiments has been concerned with the "arm-first" formation of polybutadiene or polyisoprene stars. The initiation of the polymerization of divinylbenzene by butadienyl or isoprenyl sites is slow, and this has two important consequences: on one hand, a high proportion of linear polybutadiene or polyisoprene remains, even after very long reaction times and, on the other hand, the cores are very large and of high functionality. From this investigation it follows that the "arm-first" method does not lend itself well to the synthesis of polydiene stars free from residual homopolymer.
(B) The "core-first" method consists in preparing a multifunctional organometallic initiator and in employing it for the initiation of a monomer (styrene, dienes, vinylpyridine, (meth)acrylic esters) the polymerization of which will form the arms. The difficulty lies in obtaining homogeneous solutions of multifunctional cores. The procedure followed is the anionic polymerization of divinylbenzene at high dilution, the divinylbenzene/initiator molar ratio being chosen within narrow limits. The disadvantage of this method is the very high mass- and functionality-polydispersity of these samples of star-shaped polymer; its advantage is that it allows the functionalization of the arms at their end and the synthesis of stars with block arms.
While the "core-first" method yields satisfactory results with styrene in polar medium, the present inventors have been able to show, during preliminary tests performed on isoprene, that this method is difficult to use with dienes in apolar medium, this being for two reasons:
the physical associations between active sites, in apolar medium, result in the solidification of the reaction medium at an early stage of the polymerization, making stirring virtually impossible in a medium;
if the initiator cores contain residual double bonds (which is demonstrated by the results described above), chemical bridges can also form, thus resulting in the formation of a polymer network.
(C) The "double star" method of synthesis of macromolecules takes place in three stages ("in-out" method). Advantage is taken of the fact that, during the "arm-first" synthesis, each core contains a number of organometallic sites which is equal to the number of arms which surround it. These sites can subsequently serve for the initiation of the polymerization of a second monomer, which results in the formation of a second generation of arms.
The second-generation arms thus formed carry an active end site and can therefore be functionalized at a chain end. Their average length is given by the molar ratio monomer/active sites, and their number is substantially equal to that of the first-generation arms. The latter are preferably chosen to be short, so that the second-generation arms constitute the main part of the polymer material.
This method has been successfully applied to the synthesis of double stars of polystyrene-polyoxyethylene, polystyrene-poly(methyl methacrylate), polystyrene-poly(tert-butyl acrylate) and polystyrene-polyvinylpyridine, the first-generation polystyrene arms being in most cases of low mass if they are intended exclusively to ensure the protection of the polydivinylbenzene cores which contain the active (polyfunctional initiator) sites.
The unsaturations which could continue to exist within the cores of the polystyrene "primary" stars do not interfere with the synthesis of the double stars to which reference has just been made. On the other hand, if the second-generation arms result from the polymerization of weakly electrophilic monomers, these double bonds are responsible for the formation of intermolecular bridges. This is the case with styrene and, in general, with vinylaromatic monomers and dienes. Various attempts at synthesis, using the "in-out" method, of double stars whose second-generation arms are made of polystyrene, polyisoprene or polybutadiene, have, however, ended in failure because of the chemical crosslinking which is seen as the irreversible solidification of the reaction mixture. This bridging is attributable to the reaction of a growing active site, situated at the end of an arm, with a residual double bond belonging to the core of another molecule.
It has therefore been found advantageous to investigate processes for forming double stars whose second-generation arms would include vinylaromatic and/or polydiene polymer blocks.
The present inventors have concentrated on the problems linked with the insolubility of the multifunctional initiators in apolar medium and with the chemical crosslinking which can take place when a living crosslinked core is employed as multifunctional initiator. They have first of all studied the kinetics of formation of the polystyrene "primary" stars in apolar medium and devoted themselves to measuring the proportion of residual double bonds in the cores, as a function of the reaction time. The presence of residual unsaturations has been confirmed, even after very long reaction periods, corresponding to the final value of the molecular mass of the primary star.